THE CLASSIFICATION OF ANIMALS. 13 



called, is distinctly an invertebrate animal with no notochord 

 and a ventral nervous system. 



How, then, can we distinguish these from true vertebrates 1 

 At no time do they possess a brain or cranium as we see in the 

 higher animals. They are called, tlierefore, Acrania, to dis- 

 tinguish them from all the other vertebrates, which are known 

 as the Craniota. This was the most generally adopted primary 

 grouping of animals, into Invertebrata and Vertebrata ; but for 

 many reasons a more recent classification has many advantages 

 over it. This latter is based primarily on the cell-structure 

 of the animal. By it the whole animal kingdom is divided 

 into two primary groups, known as the Protozoa and Metazoa. 



The Protozoa are those animals of extremely simple organisa- 

 tion, and whose bodies are composed of a single cell. 



The Metazoa constitute a group that includes the majority of 

 animals. These are built up, not of one cell or a few cells, 

 but of countless numbers of cells, which form the complicated 

 animal tissues — muscular, nervous, connective, &c. This divi- 

 sion will be found to contain both invertebrates and vertebrates. 



The Protozoa form the first group of animals, the lowest 

 organisms, single-celled creatures, which are, nevertheless, im- 

 portant to us, as many of them produce diseases, such as liver- 

 rot in rabbits, malarial fever, sleeping sickness, redwater in 

 cattle, psorospermosis of the skin, &c., in man and his- domestic 

 animals. 



The multicellular animals, or Metazoa, are divided into the 

 following groups, called classes : — 



1. Godenterata, or Jellyfish, Polyps, Sea-anemones, Corals, &c. 



2. EcMnoderniaia, or Starfish, Sea-urchins, and the nearly 



extinct Sea-lilies. 



3. Vermes, or Worms. 



4. MoUusca, or Shells. 



5. Arthropoda, or the Jointed-limbed animals, as Insects, 



Spiders, Scorpions, and Crabs, &c. 

 The above are all Invertebrate Metazoa. The Sponr/idii', or 



